This invention relates to wireless communication systems, and more particularly, to wireless communication systems using multiple antennas at the transmitter and receiver.
Wireless communication systems that use multiple antennas at the transmitter and multiple antennas at the receiver, so-called multi-input multi-output (MIMO) systems, can achieve dramatically improved capacity compared to single antenna systems, i.e., systems that have a single antenna at the transmitter and a single antenna at the receiver. As shown in FIG. 1, at transmitter 120 of a MIMO wireless communication system 100, primitive data stream 130 (i.e., the data stream to be transmitted) is processed, using well-known techniques, to form space-time coded signals. Particularly, transmitter 120 divides the primitive data stream into data sub-streams. The data sub-streams are channel encoded, interleaved in space and time, and multiplexed onto the transmit antennas to produce the space-time coded signals. Typically, each of the space-time coded signals is transmitted over a different one of the transmit antennas 115-1, 115-2, 115-3, and 115-4.
The signals emanating from the transmit antennas arrive at receive antennas 125-1 and 125-2. The received signal at each of the receive antennas is typically a superposition of each of the transmitted signals. Though the transmitted signals interfere with each other, received signals are processed in receiver 140 to separate out and then decode these superpositioned signals.
In a receiver that uses iterative decoding, such as the one shown in FIG. 1, the received signal is processed in MIMO detector 150 and channel decoder 155 several times to reduce the number of bit errors in the decoded signal. First the symbols of the received signals are processed in MIMO detector 150. MIMO detector 150 processes the received signals received over receive antennas 125-1 and 125-2, one vector symbol at a time, to determine the individual transmitted signals. (A vector symbol is a vector, each of whose components is a symbol received on one of the receive antennas during the same symbol period; a symbol period being a time period whose duration is one symbol.) During the processing of the symbols, MIMO detector 150 also digitally demodulates the signals, i.e. maps them from symbol space into bit space, to obtain soft value bits. Soft value bits are bits whose value conveys both whether the bit is a one or zero and an indication of the certainty with which this is known. A block of the soft value bits is deinterleaved in deinterleaver 152 and then processed in channel decoder 155. (At output 162 of channel decoder 155, the values of the soft value bits are just their values as decoded by channel decoder 155 on the last pass through. That is their values at the input of the channel decoder are, for example subtracted out.) The soft value bits as decoded by channel decoder 155 are interleaved in interleaver 157 and fed back as an input to MIMO detector 150 to again process the bits using the new values of the soft value bits. (Similarly to channel decoder 155, MIMO detector 150 includes subtraction operation to obtain the values of the soft value bits as detected by MIMO detector 150.) The processing of the soft value bits in the MIMO detector using the values obtained from their last pass through the channel decoder, and visa versa is repeated several times. The resulting bits at output 162 include both their values as decoded by channel decoder 155 on this last pass through and the values of the soft value bits at the input of the channel decoder. The resulting bits can be further processed in receiver 140, such as by, for example, demultiplexing them, to obtain reconstructed primitive data stream 195.
The channel coding is used for error correction. That is, the channel decoder may be able to recover bits that arrive at the receiver in error due to noise and/or interference. Conventionally, a strong channel code, such as a turbo code, is used. (A strong channel code is one that in most environments can achieve either 1) the same bit error rate at a lower signal-to-noise ratio as a weaker code that has the same redundancy or 2) a reduced bit error rate at the same signal-to-noise ratio as a weaker code that has the same redundancy.) In single antenna systems an increase in the strength of the code typically results in a decrease in the number of erred bits in the decoded signal. However, recent research has shown that an increase in the strength of the channel code does not necessarily result in a reduction in the errors in the decoded signal at the receiver when the number of antennas at the transmitter is larger than the number of antennas at the receiver.